1. Field of the Invention
Embodiments of the present invention generally relate to interrogation of optical components and, more particularly, to techniques and apparatus for avoiding overlap of reflections from different arrays of optical elements along the same waveguide when interrogated using wavelength-swept light.
2. Description of the Related Art
Many optical components have a characteristic wavelength that may be found by interrogating the optical component with an optical source capable of producing light at various wavelengths over a fixed range or bandwidth. For example, fiber Bragg gratings (FBGs) (typically formed by photo-induced periodic modulation of the refractive index of an optical waveguide core) are highly reflective to light having wavelengths within a narrow bandwidth centered at a wavelength generally referred to as the Bragg wavelength. Because light having wavelengths outside this narrow bandwidth is passed without reflection, Bragg wavelengths can be determined by interrogating a Bragg grating with a light source swept across a bandwidth that includes the Bragg wavelength and monitoring the reflected optical power spectrum at a receiver unit. Because Bragg wavelengths are dependent on physical parameters, such as temperature and strain, Bragg gratings can be utilized in optical sensor systems to measure such parameters.
In these and a wide range of other types of optical systems, the measurement of a characteristic wavelength of an optical component to great accuracy (and/or with great repeatability) is important to system performance. Two significant parameters determining the error of any such measurement are the signal-to-noise ratio (SNR) and effective integration time of the measuring system. SNR is dependent of many factors including received optical power, optical-source noise, and receiver noise. The effective integration time is dependent on overall averaging time and the proportion of that time which is producing useful signals at the receiver unit. Improving these two parameters can improve characteristic wavelength measurement repeatability and accuracy.
Conventional swept-wavelength Bragg grating interrogators are limited in the number of sensors that can be interrogated on a single fiber by the optical bandwidth of the source because only wavelength division multiplexing (WDM) is used to interrogate the sensors. Many of the applications using Bragg grating sensors can be improved by increasing the number of sensors in the system.
In some cases, this problem has been addressed by increasing the optical bandwidth of the light source to enable more sensors to be multiplexed on the optical fiber using WDM.
In other cases, a swept-wavelength interferometry technique has been used. This technique may allow time division multiplexing (TDM) of grating sensors; however, the swept-wavelength interferometry technique may suffer from a limited spatial window range as well as limited wavelength resolution. In yet another approach, time-gated lasers have been used to TDM Bragg grating sensors.